4.2 A Digression--Truth Values

We have mentioned truth and "true and false" a few times now; however,
we have yet to give a clear definition of what truth values are in Perl.

Every expression in Perl has a truth value. Usually, we ignore the
truth value of the expressions we use. In fact, we have been ignoring
them so far! However, now that we are going to begin studying various
control structures that rely on the truth value of a given expression,
we should look at true and false values in Perl a bit more closely.

The basic rule that most Perl programmers remember is that 0, the
empty string and undef are false, and everything else is true.
However, it turns out that this rule is not actually completely
accurate.

The actual rule is as follows:

Everything in Perl is true, except:

the strings "" (the empty string) and "0" (the
string containing only the character, 0), or any string expression that
evaluates to either "" (the empty string) or "0".

any numeric expression that evaluates to a numeric 0.

any value that is not defined (i.e., equivalent to
undef).

If that rule is not completely clear, the following table gives some
example Perl expressions and states whether they are true or not:

Expression

String/Number?

Boolean value

0

number

false

0.0

number

false

0.0000

number

false

""

string

false

"0"

string

false

"0.0"

string

true

undef

N/A

false

42 - (6 * 7)

number

false

"0.0" + 0.0

number

false

"foo"

string

true

There are two expressions above that easily confuse new Perl
programmers. First of all, the expression "0.0" is true. This
is true because it is a string that is not "0". The only string
that is not empty that can be false is "0". Thus, "0.0"
must be true.

Next, consider "0.0" + 0.0. After what was just stated, one
might assume that this expression is true. However, this expression is
false. It is false because + is a numeric operator,
and as such, "0.0" must be turned into its numeric equivalent.
Since the numeric equivalent to "0.0" is 0.0, we get the
expression 0.0 + 0.0, which evaluates to 0.0, which is the
same as 0, which is false.

Finally, it should be noted that all references are true. The topic of
Perl references is beyond the scope of this book. However, if we did
not mention it, we would not be giving you the whole truth story.